Regular and reliable climate observations with measurement and
calculation of principal climatic indices were started only about 150 years ago,
when organized and consistent global ocean and atmosphere observation systems
were initiated.

The averaged surface air temperature anomaly (dT) is widely
recognised to be the most important index characterising the global climate
changes including "global warming" (Bell et al. 1998; Anisimov and
Polyakov 1999).

Another single index of climate change is the Atmospheric
Circulation Index (ACI) that characterizes the periods of relative dominance of
either "zonal" or "meridional" transport of the air masses on the hemispheric
scale. ACI, also known as the Vangengeim-Girs Index, has been calculated from
data on basic atmospheric activities in the Atlantic-Euroasian region for more
than 100 years (Girs 1974).

George Vangengeim, the founder of ACI, is a well-known Russian
climatologist. The Vangengeim-Girs classification is the basis of the modern
Russian climatological school of thought. According to this system, all
observable variation in atmospheric circulation is classified into three basic
types by direction of the air mass transfer: Meridional (C); Western (W), and
Eastern (E). Each of the above-mentioned forms is calculated from the daily
atmospheric pressure charts over northern Atlantic-Eurasian region. General
direction of the transfer of cyclonic and anticylonic air masses is known to
depend on the distribution of atmospheric pressure over the Atlantic-Eurasian
region (the atmosphere topography).

The recurrence of each circulation form (W, E, or C) during
the year is expressed in days. Total annual duration of the three circulation
forms sums therefore to 365 (or 366 in a leap year). The index is defined by the
number of days with the dominant form of atmospheric circulation. It is more
conveniently expressed as an anomaly (actual data minus the long-term average).
The sum of anomalies can be displayed in a chart of the so-called integral curve
of the atmospheric circulation. The annual sum of the occurrence of all
circulation anomalies is equal to zero: (C) +(W) + (E) = 0.

The periods dominated by any single form of atmospheric
circulation have alternated with a roughly 30-year period for the last 100
years. These periods were named "Circulation epochs". These may be pooled into
two principal groups: meridional (C) and combined "latitudinal" epochs (W + E):
(W + E) = - (C)

Meridional (C) circulation dominated in 1890-1920 and
1950-1980. The combined, "zonal" (W+E) circulation epochs dominated in 1920-1950
and 1980-1990. Current "latitudinal"(WE) epoch of 1970-1990s is not completed
yet, but it is coming into its final stage, and so the "meridional" epoch
(C-circulation) is now in its initial stage. (It will be useful for the reader
to note here the relation that shows that the "transition" from C to W-E is
continuous, and the equation balances to 100%, in the form of a simple graphic
without any other variables included).

It was found that "zonal" epochs correspond to the periods of
global warming and the meridional ones correspond to the periods of global
cooling. (Lamb 1972; Lambeck 1980). The generalised time series on the
atmospheric circulation forms for 1891-1999 were kindly placed at our disposal
by the Federal Arctic and Antarctic Research Institute (AARI) in St. Petersburg
(Russia). This is also consistent with the theories and observations described
by Leroux (1998).

The third important index is Length of Day (LOD) - a
geophysical index that characterizes variation in the earth rotational velocity.
Full time series of LOD cover more than 350 years, with the most reliable data
obtained in the last 150 years (Stephenson and Morrison 1995). The long-term LOD
dynamics is in close correlation with the dynamics of the main commercial fish
stocks (Klyashtorin and Sidorenkov 1996).

LOD Index is calculated as a difference of two values: actual
(astronomical) length of day and the standard one. The continual registration of
LOD and publication of the corresponding data is carried out by the
International Time Bureau in Paris. The correlation coefficients were calculated
using non-smoothed data ranges. Smoothed values, obtained using a Gaussian
filter, were only used to improve the curve plots in the figures. Statgraphics
(1988) software was used to detrend the time series.

Let us compare the dynamics global geophysical (LOD) and
climatic indices for the last 150 years. For a more convenient visual comparison
of the dT, ACI and the earth rotation velocity dynamics, the negative (-LOD)
instead of the directly calculated LOD values was used. The dynamics of -LOD, dT
and ACI for the last 140 years can be viewed as multi-decadal fluctuations
against the background of age-long trends (Figure 2.1).

Global dT is known to have an ascending linear trend 1861-1999
with the increment of 0.059o per 10 years (Sonechkin 1998).

Our planet as a whole conserves its angular momentum except
for the known effects of external torque associated with the lunar-solar tide,
which induces a gradual decelerating of the earth rotation velocity at a rate
corresponding to the increase in the astronomic length of day (LOD) by about 1.4
millisecond per century (Munk and McDonald 1960). That is, -LOD has had a
descending linear trend with the increment about 0.14 ms per decade (Figure
2.1). Different age-long linear trends of LOD and dT make it difficult to
compare the dynamics of multi-decadal fluctuations of these indices. Figure 2.2a
presents both -LOD and dT ranges, with the linear trends removed by fitting of
linear regression and using a "detrending" procedure (Statgraphics
1988).

When detrended, the graphs of -LOD and dT are very similar in
shape, and it is clear that -LOD runs several years ahead of dT, especially in
its maxima. Shifting the -LOD curve by 6 years to the right (Figure 2.2b)
results in almost complete coincidence of the corresponding maxima of the early
1870s, late 1930s, and middle of 1990s (Klyashtorin et al. 1998).

The similarity in the dynamics of detrended -LOD and dT and
ACI indices is clear: Large fragments of the curves are much alike not only in
general shape, but in detail as well. In general, the long-term dynamics of both
dT and -LOD have roughly a 60 year periodicity. The global climate system was
reported to oscillate with a period of 65-70 years since 1850 (Schlesinger and
Ramankutty 1994). The same 60-70 year periodicity has also been characteristic
for the long-term dynamics of some climatic and biological indices for the last
150 years (Klyashtorin 1998).

Similarity between the -LOD and dT dynamics makes it possible
to assume the existence of some common factors inducing and controlling the
observable synchrony in geophysical (LOD) and climatic (dT) indices variation.
The atmospheric circulation as a whole is strongly driven by the Equator-Pole
Temperature Meridional Gradient. Greater warming in the polar region weakens
this gradient in the lower troposphere, which leads to a general weakening of
surface winds (Lambeck 1980).

The long-term dynamics of the atmospheric pressure fields over
the Northern hemisphere during the last 90 years are characterised by the
alternation of approximate 30-year periods ("circulation epochs") with relative
dominance of either zonal or meridional atmospheric circulation (Dzerdzeevski
1969; Girs 1971; Lamb 1972; Lambeck 1980).

The first type, zonal circulation, is characterised by
increasing intensity of the zonal circulation at all latitudes and pole-ward
shift of the wind intensity maximums. The circulation is accompanied somewhat by
a decrease in the overall range of surface-air temperature between the equator
and poles and by an overall increase in the mean global surface-air
temperatures. Ocean-surface temperatures tend to increase in high latitudes. The
second type, meridional circulation, is characterised by weakening in zonal
circulation, shift of the main atmospheric streams toward lower latitudes, and
overall decrease in global temperature (Lamb 1972). Both easterly and westerly
winds increase during the zonal type of circulation and both decrease in the
periods of the meridional type of the circulation (Lambeck 1980).

Atmosphere is the most variable component of the global
geophysical system exchanging relatively large proportions of its angular
momentum with the solid earth below compared with other components (Salstein
et al. 1993). A number of publications suggest that seasonal,
inter-seasonal and inter-annual variations of the earth's rotation velocity are
directly proportional to the relative angular momentum at the atmosphere, which
primarily depends on the velocity of zonal winds (Langley et al. 1981;
Rosen and Salstein 1983; Robertson 1991). On longer time scales, changes in LOD
are correlated with the El Niño Southern associated with ENSO events
(Salstein and Rosen 1986; Dickey et al. 1992a, b)

A formal derivation of the dynamic relation between the
atmosphere and solid earth make it possible to calculate the changes in the
earth's rotational velocity from the large-scale distribution of the atmospheric
pressure and dynamics of the wind fields (Barnes et al. 1983). These data
are available from several of the world's weather services (Salstein et
al. 1993).

Thus, on a wide range of time scales from several days to
years, there is an agreement between the dynamics of the angular momentum in the
atmosphere and solid earth, which come into view as small but important changes
in the rotation of the planet.

It is conceivable that the multi-decadal fluctuations of the
earth's rotation velocity results from the redistribution of the angular moment
between the atmosphere and solid earth due to the alternation of multi-decadal
epochs of "zonal" and meridional atmospheric circulation.

It was shown (Lamb 1972; Lambeck and Cazenave 1976), however,
that the observable changes in speed and direction of the air mass transfer may
explain seasonal and annual, but not multi-decadal, LOD variations. Only 10% of
the long-term LOD variation can be explained by the observable changes in
atmospheric circulation. The calculations suggest that the average speed of
zonal winds would have to be an order of magnitude larger than they are to
explain the remaining 90% of the LOD changes. It seems improbable that some
other unevaluated meteorological factors could provide additional explanation.
Therefore, the hypothesis that the climatic changes are a consequence of the LOD
changes should be rejected (Lambeck 1980).

2.1 SUMMARY

A phenomenon of close correlation between the main climatic
index dT and geophysical index (-LOD) still remains an intricate puzzle of
geophysics. Another challenging puzzle is the observable 6-year lag between the
detrended run of dT and -LOD. Taking into account this lag, the LOD observations
can be used as a predictor of the future climatic trends. Even without a
mechanism for a causal relationship between the detrended climatic (dT) and
geophysical (LOD) indices, the phenomenon of their close similarity for the last
140 years makes LOD a convenient tool to predict the global temperature anomaly
(dT) for at least 6 years ahead.

Lambeck and Cazenave (1976) pointed out a high probability of
an increasing global temperature trend in the 1970s and 1980s. Their prediction
was correct.

Spectral density analysis of the LOD time series for 1850-1998
revealed clear, regular fluctuations with an approximate 60-year period length
(see below). The multidecadal maxima of LOD took place in the early 1870s and
mid-1930s, and the next maximum is likely to fall early in 2000. Based on this
multidecadal periodicity of LOD, and the fact that LOD runs ahead of dT by 6
years, a gradually descending dT may be expected around 2005.

Although the interdependences between dT, ACI and -LOD may be
complex, the empirical geophysical index (LOD) may still serve as a general
predictor of future climate changes. This gives an opportunity to forecast
climate change and, therefore, stock and catches of commercial species, which
are related to climate.